1. Technical Field
The present invention relates to a method of aligning a laser diode array and optical fiber array and a method of fixing the components of them, in a laser diode and a semiconductor laser module where a laser beam emitted from the laser diode is optically coupled to an optical fiber, relates to a semiconductor laser module using the aligning method and fixing method, and relates to a method of manufacturing the semiconductor laser module.
2. Background Art
As the output of a laser diode recently increases, a laser processing apparatus using the laser diode as a light source or excitation source is widely used for various material processing such as welding, deposition, cutting, and modification.
In order to efficiently transmit a laser beam output from a laser diode to an optical fiber, a semiconductor laser module using the laser diode generally has the following configuration:                the laser diode and optical fiber are precisely aligned, and are fixed to respective holding members so as to prevent deviation of alignment.These holding members are fixed to predetermined positions using one of laser welding, soldering, and an adhesive, or a combination of them (for example, Unexamined Japanese Patent Publication No. 2002-341196).        
As a semiconductor laser module for industrial use, a semiconductor laser module including a laser diode array where a plurality of light emitters is arranged in parallel and at least as many optical fibers as the light emitters of the laser diode array are arranged in parallel is disclosed (for example, U.S. Pat. No. 4,818,062 and Unexamined Japanese Patent Publication No. 2005-010374).
Recently, a semiconductor laser module is developed where the luminance at the light intensity per light emitter of a laser diode is high, about 10 W, and the fiber output is 100 W or higher. In the optical fiber array of the semiconductor laser module of high output, as many multimode optical fibers as the light emitters are fixed on a substrate in parallel with a pitch that is equal to the interval between the light emitters. Here, the multimode optical fibers have a core radius that is equal to or larger than the width of each light emitter of the laser diode array. The optical fiber array is aligned to the laser diode array so that the light intensity output from the optical fiber array is the highest, and is fixed on a casing by various fixing methods discussed above. The alignment of the laser diode array and optical fiber array is performed in a lump by attaching the optical fiber array to a precise fixture or precise stage (for example, Unexamined Japanese Patent Publication No. 2004-326004 and Unexamined Japanese Patent Publication No. 2008-186035).
The alignment of the laser diode array and optical fiber array is performed with reference to a total of six axes. In other words, the alignment includes alignment in the X axis, Y axis, and Z axis directions of the optical fiber array and alignment about rotation angles Xθ, Yθ, and Zθ about the X axis, Y axis, and Z axis, as shown in Unexamined Japanese Patent Publication No. 2005-010374 or the like.
Generally, the alignment is performed by monitoring the light intensity input to the optical fibers and adjusting the six axes so that the highest light intensity is obtained. Especially regarding the optical fiber array, the alignment is performed with reference to each axis using the sum of the light intensities of a plurality of optical fibers or all optical fibers or using the light intensities of selected optical fibers at typical positions such as the center or both ends of the optical fiber array
The laser beam output from the light emitter of the laser diode spreads in the thickness direction (fast axis) and the width direction (slow axis) at different spread angles. Therefore, when the laser diode and optical fiber array are aligned by collimating the laser beam, the tolerance of the alignment position capable of providing a desired optical coupling is different between the axes dependently on the beam form.
Especially, it is known that the alignment position on the Y axis corresponding to the thickness direction of the laser diode is sensitive to the optical coupling efficiency and light beam quality (e.g. NA (numerical aperture)) after light guide of the optical fibers. In addition, the alignment about Zθ includes the Y-direction component in the alignment of the laser diode array and optical fiber array, so that the alignment about Zθ is more complicated.
Recently, the semiconductor laser module for industrial use especially requires not only higher optical coupling efficiency but also a high-quality spread angle (NA) of light output beam, so that the appropriate range of the alignment position is narrow. For example, when the optical coupling efficiency of the laser diode array and optical fiber array is about 90% and the output beam quality from each optical fiber is about NA 0.12, the range on the Y axis satisfying these values is narrow, namely about 1.5-2.0 μm
Thus, the alignment of the laser diode array and optical fiber array requires a high-function and high-accuracy aligning mechanism. This aligning mechanism, for example, is a sensing means and aligning procedure for determining the alignment position allowing the desired optical coupling, or a precise stage for performing six-axis control and performing positioning at a tolerance required for the alignment position.
Furthermore, the alignment requires an optical fiber array fixing method for keeping the alignment position allowing the desired optical coupling, and a member configuration appropriate for the fixing method. By using an aligning apparatus including these components, a high-performance and high-reliability semiconductor laser module can be assembled and manufactured.
As in the aligning apparatus disclosed by Unexamined Japanese Patent Publication No. 2008-186035, various methods of performing precise alignment so that the optical coupling between the laser diode array and optical fiber array becomes desired are proposed. Many methods have been commercialized. Also in the aligning mechanism, the function and accuracy of the precise stage are recently, extremely improved, and hence the six-axis control and the positioning of the optical fiber array can be performed in units of sub-micrometers without problems.
While, the optical fiber array fixing method and the member configuration appropriate for the fixing method have many problems. In the fixing method of an optical member such as an optical fiber array, an adhesive is generally used. This is because an adhesive works as a filler for filling the clearance between the optical member and a member or casing for holding it, hence the positional relationship between the members is allowed to be relatively unclear, and the handling and assembling are made easy.
However, the volume varies in the contraction direction when the adhesive hardens, so that the optical fibers can deviate from the alignment position, the optical output and light beam quality can reduce, and hence the yield of the products can reduce. In the actual manufacturing, extremely much study time and know-how are required for correcting the deviation corresponding to the contraction from the alignment position in consideration of the hardening and contraction of the adhesive, for example. The semiconductor laser module after the manufacturing has problems in securing the long-term reliability, such as aging degradation of a bonding section, deviation from the alignment position, and drop of a member.
As discussed above, the recent semiconductor laser module for industrial use requires higher output, higher efficiency, and higher beam quality, so that the tolerance of the alignment position is extremely narrow. When the optical fiber array is used, the total volume and weight are several times those when only one optical fiber and micro-optics are used. Inevitably, the amount of used adhesive also increases, and the possibility that the above-mentioned problems related to the adhesive become more remarkable is high.
As the fixing method that compensates for the problems caused by the hardening/contraction and aging degradation of the adhesive, laser welding is used. The laser welding is a process of locally heating and melting/bonding a part to be fixed for an extremely short time, and the aging degradation that can occur in the adhesive hardly occurs.
However, the laser welding is local heating in principle. The laser welding is therefore sensitive to the clearance between the members to be welded and the accuracy of the mating surface, and strict management of the state between the members to be fixed is required differently from the adhesive. When the clearance between the members and the accuracy of the mating surface are insufficient even if the laser welding/fixing is enabled, the optical fibers can deviate from the alignment position due to coagulation/contraction from the molten state. In other words, when the clearance between the optical fiber array and the holding member and the accuracy of the mating surface are secured sufficiently in fixing the optical fiber array, fixing accuracy and reliability that cannot be obtained by the adhesive can be obtained. The laser welding requires an additional stage adjusting mechanism for mating the laser welding joint surface in addition to the six-axis required for alignment, and hence an extremely complicated aligning apparatus is required.
An example of the semiconductor laser module using an optical fiber array, which includes the aligning method, is disclosed by Unexamined Japanese Patent Publication No. 2005-010374, for example. The following method is proposed:                a guide plate for positioning the optical fiber array is previously disposed between the laser diode array and optical fiber array, and is used as the reference for positioning (for example, the optical fiber array is butted on the guide plate), thereby reducing the number of alignment axes.        
When the laser diode array is mounted on a sub-mount, a mounting displacement or rotation displacement of several micrometers inevitably occurs because the size of the laser diode chip is relatively large, namely about several millimeters per side. The relationship in alignment position between the laser diode array and optical fiber array is in the order of micrometers. Therefore, the individual difference of the mounted position of the laser diode array must be corrected and absorbed in units of micrometers using the guide plate. It is extremely difficult that this work is performed by the individual semiconductor laser module. When the laser welding is used as the fixing method, it is also difficult that the clearance between the welded parts and the accuracy of the mating surface are secured by the individual semiconductor laser module.